One of the key issues in yield farming is to determine the best farming, lending and liquidity providing opportunities. Unified interface allows users to enter this yield sources just in a few clicks. You just need to approve tokens and deposit them directly in yield source's smart contract. LP token will be generated and credited in a single transaction, saving gas as well. Meaning you don't need to enter yield source's interface in order to accumulate it.
The same logic is used for the withdrawal. No messing up with LP tokens - underlying tokens will be transfered directly to your wallet.
Nevertheless if you do have LP tokens already you can enter protocol with it.